Difficulty: Easy

Related Topics: Bit Manipulation

Given an integer n, return true if it is a power of four. Otherwise, return false.

An integer n is a power of four, if there exists an integer x such that n == 4<sup>x</sup>.

Example 1:

  1. Input: n = 16
  2. Output: true

Example 2:

Input: n = 5
Output: false

Example 3:

Input: n = 1
Output: true

Constraints:

  • -2 <= n <= 2 - 1

Follow up: Could you solve it without loops/recursion?

Solution

Language: Java

class Solution {

    /*
    解题思路：
    1. 先判断是不是2的次方数 (n & n - 1) == 0
    2. 在判断它是不是4的次方数
    */
    public boolean isPowerOfFour(int n) {
        if (n <= 0) return false;

        int mask = 0x55555555;

        /*
          01010101
        & 00010000
        ----------
          00010000 <- not equal 0
        */
        return  (n & n - 1) == 0 && (n & mask) != 0;
    }
}